There is a requirement in many military-specified devices for power supply provisions to integrated circuits which operate in such a way as to rapidly reduce the power supply voltage to the chip so that the phenomenon of "latch-up" can not occur. The reason for this is that, under radiation conditions, very large "photo" currents can flow in the junctions of the integrated circuits such that parasitic transistors in the structure are turned on and large currents can flow, potentially destroying the device through power from its own supplies. Such photocurrents are only predictable to a limited extent, and it must be assumed that all junctions are capable of passing large reverse currents and that transistor action during these conditions is severely impaired. This means that circuits which operate during the time of radiation must do so independently of their normal parameters.
One means of radiation hardening is to minimise the number of junctions present, and this is done using oxide isolation, in various schemes already in production. However, junctions must exist for conventional device operation, and so must be protected.
In conventional integrated circuits, the resistors are as vulnerable as the transistors to photocurrents, since they have at least one large junction to substrate, and in some cases further junctions to the surrounding material. Since the resistor is often larger than the accompanying transistors, it may be more susceptible to radiation.